This invention relates to a printed circuit heating element. More specifically, the present invention relates to a printed circuit heating element suitable for heating and welding two thermoplastic tubes together.
At the present time there are a number of medical and scientific procedures which require the sterile transfer of fluids from one container to another. An example of the need for sterile docking is in continuous ambulatory peritoneal dialysis (CAPD). The CAPD patient has a tube connected to his or her peritoneal cavity via an implanted catheter. A tube from a bag of fresh dialysis solution is connected to the patient's tube. The fresh dialysis solution is drained from the bag into the patient's peritoneal cavity where it remains for about 3-4 hrs. After this treatment period, the spent dialysate is drained back into the empty bag which is then disconnected from the patient's tube. A bag of fresh dialysis solution is then connected to the patient's tube and the procedure repeated. A similar need for sterile connection exists for blood bags.
U.S. Pat. No. 4,369,779, issued to Spencer on Jan. 25, 1983, discloses an apparatus and process for forming a sterile connection. The apparatus comprises a cutting means, means adapted to heat said cutting means, a pair of mounting blocks adapted to receive and hold two tubes to be joined, means to provide movement between said blocks and said cutting means to a position such that the cutting means is between said blocks and traversing where the blocks are adapted to receive tubes, means adapted to realign said blocks to a position where two different tube ends are aligned with and facing each other, and means to separate said blocks and said cutting means while urging said blocks together. The patent discloses that the cutting means can take many forms but preferably is a laminate strip constructed of an etched stainless steel ribbon having on each side an acrylic adhesive layer, an aromatic polyimide layer, an acrylic adhesive layer and a copper ribbon.
U.S. Pat. No. 4,501,951 discloses a heating element for use in the aforesaid apparatus for sterile connection consisting essentially of, as an outside layer, a folded sheet of metal having a thermal conductivity of at least about 173 watts/m.degree.K. at a 0.10 mm thickness and a tensile yield strength of at least about 34.times.10.sup.4 kPa at a 0.10 mm thickness, a resistor disposed inside the fold of said folded sheet of metal; and a layer of dielectric adhesive, stable to about 260.degree. C., between inner surfaces of said folded sheet of metal and surfaces of said resistor, thereby bonding the resulting structure together. The application further discloses that preferably the resistor is an etched foil resistor made from stainless steel.
Resistance heating elements for use in the aforesaid sterile connection apparatus can be reused if properly cleaned and if they are still structurally intact. However, improper cleaning may result in a joint of low strength and in contamination of the insides of the tubes which are welded together. Moreover, the number of times a heating element can be reused depends upon the particular element. Furthermore, the determination of whether a heating element has been properly cleaned or is structurally intact cannot be easily done by the ordinary user. Uncertainties such as these, subject the patient who is using the device to unnecessary risks of infection. A means by which these risks may be minimized, eliminated or avoided would prove advantageous.
Fuses which melt to deactivate or bypass a circuit are well known. U.S. Pat. No. 411,380 issued to Heller and U.S. Pat. No. 1,803,486 issued to Rau are typical of patents disclosing fusible elements to bypass an incandescent electric lamp. U.S. Pat. No. 2,874,248, issued to Tondat et al. on Feb. 17, 1959, discloses a multiple fuse assembly having a movable bridging contactor to engage one fuse at a time. The patent discloses that the assembly may be produced by printed circuit technology or by spraying a paint containing powdered metal through a suitable mask to produce the desired outline on an insulating board, and then drying and baking the resulting coating.
U.S. Pat. No. 3,249,800, issued to Huber on May 3, 1966, discloses a fast acting switch wherein a small metal wire is exploded electrically and rapidly to pulse a circuit by electrically disintegrating the wire explosively to form a rapidly expanding, current carrying, low impedance conducting medium in the gap between two closely spaced conductors so as to actuate and conduct the flow of current between the conductors.
U.S. Pat. No. 4,169,254, issued to Kennedy et al. on Sept. 25, 1979, discloses a manually actuable electric switch having a fusible element which melts on subjection to a predetermined temperature to discontinue the flow of electric current through the switch. A manually operable plunger normally used to open and close the switch is mechanically connected to the stem of the switch through the fusible element. Melting of the element severs the mechanical connection between the plunger and switch thereby deactivating the switch.